During the processing of flat items of mail, in particular letters, postcards and so on, by postal delivery companies, the task arises of secure and rapid application of labels to flat items of mail. One example of this is the automatic forwarding of items of mail. In this case, items of mail to be forwarded are sorted out and addressed in accordance with predefined data, which is stored in a database. These items of mail have a label stuck to them, which covers both the old address and a barcode possibly applied to the surface of the item of mail. The label is then provided with a new barcode and the appropriate new address. The application of the label is in this case carried out in apparatus which is integrated into automatic letter distribution systems. The arrival of items of mail in such distribution systems is different in terms of format, weight and thickness. The items of mail are conveyed in such systems at, for example, speeds of 3.6 m/sec, which places high requirements on the speed at which the application of the labels must be carried out, and also on the exact positioning of the labels. Furthermore, the handling and, in particular, the transport of the labels to the surface of the items of mail represents a general problem if the labels have a self-adhesive surface.
U.S. Pat. No. 5,200,007 describes an apparatus for the application of labels to flat items of mail, which are conveyed in separated form standing on edge along a conveying path by means of a conveying apparatus. It has a label conveying apparatus for conveying the labels, which are located on a substrate, said apparatus being controlled by a sensor device for determining the front edge of the item of mail and a pressure and applicator apparatus for applying/pressing the labels onto the surface of the items of mail. In this apparatus, the labels are printed with distribution information. Labeling at high transport speeds of the items of mail is not possible with this.
A labeling apparatus has also been disclosed in which the address labels are applied to items of mail by means of a round applicator drum which, in its surface, has openings with a controllable vacuum to hold the labels temporarily. The feeding and the cutting of the label webs is carried out in such a way that labels without addresses are not applied (U.S. Pat. No. 4,421,587).
DE 36 22 502 A1 describes a labeling head machine in which the labels on the labeling head, which presses the labels onto the object, are held with the aid of openings in the surface of the head, which are connected to a vacuum source.
If the label can be pushed onto the pressure and applicator drum only when the latter is at rest, during each application operation it is necessary for the drum to be accelerated from a standstill to a peripheral speed corresponding to the transport speed of the flat objects, at which speed the label is applied to the object transported past the pressure and applicator drum. The pressure roller and applicator drum are then braked in such a way that they come to a standstill in the initial position again after one revolution.
On account of various influences, for example belt expansion of a top belt system for transporting letters, the transport speed at which the letters are transported past the pressure and applicator drum varies continuously.
If the respective label is always to be placed accurately at a predefined point on the letter, in spite of the speed fluctuations, then a great deal of expenditure on open-loop and closed-loop control is needed for this.